I would think a small screw driver with a tapered blade would work, about 1/8 wide. Oil the plunger good and work it several times to get the lube in the internals. Slide the screw driver in till it compresses the plunger most of the way then install. The trick will be to have a small enough driver so as the plunger gets into the groove area before the driver blade stops the movement.
Lots of smart people here and a several of them have done it many times, they may have better methods or descriptions.